The present invention relates to voltage converters and, more particularly, to DC-to-DC voltage converters wherein a plurality of converters are serially connected across a relatively high voltage source to produce a relatively low voltage, high current output.
In various applications wherein low voltage, high current requirements exist, the available source voltage is often too high a magnitude to permit utilization of a single solid-state converter. For example, electric transit vehicles normally operate from a 600 volt direct current (DC) source since that voltage is necessary for the DC motors which propel the vehicle. However, the controls, the lighting and the heating systems on such vehicles typically require a lower voltage in the range of 37.5 volts DC. Although conversion of the 600 volts to 37.5 volts can be accommodated by motor/generator sets or by silicon control rectifiers and silicon steel laminated transformers, such units have historically been both large, heavy and inefficient.
In an effort to reduce the size and weight of converters, conversion systems employing transistor circuits have been developed. However, transistor circuits tend to be limited in their voltage handling capability. Accordingly, it is known to connect several transistorized converter circuits in series across a relatively high magnitude voltage source and to divide the high magnitude voltage into a plurality of smaller increments by use of a plurality of capacitors serially connected across the voltage source with each capacitor being also connected across the power input terminals of a corresponding one of the converter circuits. In this manner each converter circuit is exposed to a predetermined fraction of the total source voltage.
The solid-state converters are necessarily switching converters and in the case of serially connected converters include transformers which enable their multiple output currents to be summed. In the simplest type of converter, a single switching device cyclically connects a transformer primary winding across a power source so that power is inductively coupled to the converter output via the transformer secondary winding. Since it is desired to actually produce a DC output voltage from the converter with minimum ripple content, the transformer and switching device must be capable of operating at relatively high frequency. However, the converter cost escalates appreciably with any increase in the required capability of the components to operate at higher frequencies.
In order to reduce the required frequency capability, full wave converters are commonly employed. However, the full wave converter circuits have potential overlap problems, i.e., simultaneous conduction of switching devices such as transistors at the end of each cycle or simply due to noise may cause high thermal dissipation or failure of a transistor. An alternative to the full wave converter is disclosed in U.S. patent application Ser. No. 910,330 filed on May 30, 1978 by K. K. Hedel and assigned to the General Electric Company (now U.S. Pat. No. 4,195,333). Hedel describes a staggered mode DC to DC converter which utilizes a pair of half-wave transformer coupled stages which attain the power handling advantages of the full wave converter without the potential overlap problem. Furthermore, the Hedel converter significantly reduces the input and output filter requirements, has the ability to run at greater than 50 percent duty factor to hold regulation under low source voltage conditions and incorporates a series connection of the transformer reset windings permitting operation of one of the half-wave sections to aid in the resetting of the transformer in the other half-wave section.
Using the teaching of the prior art to connect a plurality of the Hedel converters in series across the relatively high voltage source produced a converter capable of the desired voltage handling requirements. However, such an arrangement results in an imbalance of current between stages, with the exception of the final stage, which cannot be controlled without additional current regulators. Furthermore, the ripple content of the DC output voltage in such an arrangement is detrimentally effected by such current imbalance.
It is an object of the present invention to provide a DC-to-DC power converter comprising a plurality of series connected converters which is capable of operating at relatively low frequency with relatively low DC voltage output ripple. It is a further object of the invention to provide a DC-to-DC power converter comprising a plurality of series connected conversion units of the staggered mode type in which current balance is maintained without individual regulation of each conversion unit.